A flow rate of refrigerant which circulates through a refrigeration cycle apparatus is all the same in any points in a refrigeration cycle. In a cycle in which a compressor and an expander coaxially rotate, if a suction density of refrigerant passing through a compressor is defined as DC and a suction density of refrigerant passing through an expander is defined as DE, the DE/DC (density ratio) is always constant.
In recent years, attention is focused on a refrigeration cycle apparatus using, as a refrigerant, carbon dioxide (CO2, hereinafter) in which ozone destroy coefficient is zero and global warming coefficient is extremely smaller than Freon. The CO2 refrigerant has a low critical temperature as low as 31.06° C. When a temperature higher than this temperature is utilized, a high pressure side (outlet of the compressor-gas-cooler-inlet of pressure reducing device) of the refrigeration cycle apparatus is brought into a supercritical state in which CO2 refrigerant is not condensed, and there is a feature that operation efficiency of the refrigeration cycle apparatus is deteriorated as compared with a conventional refrigerant. Therefore, in the refrigeration cycle apparatus using CO2 refrigerant, in order to maintain optimal COP, it is necessary to obtain an optimal refrigerant pressure in accordance with variation in a temperature of the refrigerant.
However, when the refrigeration cycle apparatus is provided with the expander and power recover by the expander is used as a portion of a driving force of the compressor, in the cycle in which the compressor and the expander coaxially rotate, the number of rotation of the expander and the number of rotation of the compressor must be the same, and it is difficult to maintain the optimal COP when the operation condition is changed under constraint that the density ratio is constant.
Hence, there is proposed a structure in which a bypass pipe which bypasses the expander is provided, the refrigerant amount flowing into the expander is controlled, and the optimal COP is maintained (see patent documents 1 and 2 for example)
[Patent Document 1]
Japanese Patent Application Laid-open No.2000-234814 (paragraphs (0024) and (0025) and FIG. 1)
[Patent Document 2]
Japanese Patent Application Laid-open No.2001-116371 (paragraph (0023) and FIG. 1)
The patent document 1 describes that a bypass amount is increased when a pressure of a high pressure side is equal to or higher than a predetermined pressure, and the bypass amount is reduced when the pressure of the high pressure side is less than the predetermined pressure. However, a concrete determining method of the predetermined pressure for adjusting the bypass amount is not described.
Hence, it is an object of the present invention to provide a method for concretely determining this bypass amount when the apparatus includes a bypass circuit which bypasses the expander.